This invention is directed to adhesive compositions which contain multiblock hydrogenated block copolymers of vinyl aromatic hydrocarbons and conjugated dienes. More particularly, the invention is related to such adhesive compositions which contain a polymer with at least one endblock of a hydrogenated conjugated diene.
Block copolymers have been employed in adhesive compositions for many years, primarily because of their high cohesive strengths and their ability to crosslink without a chemical vulcanization step. Block copolymers such as those described in U.S. Pat. No. 3,239,478 are either linear or radial or star styrene-butadiene or styrene-isoprene block copolymers.
These conventional block copolymers when used in adhesives tend to degrade in processing and/or over time because they are unsaturated in the main rubber chain. These unsaturation sites are reactive sites which are vulnerable to attack, such as by free radicals created by oxidation, ultraviolet light or mechanical action. As a result, the polymer chain may be severed by chain scission, reducing the molecular weight and those properties which are sensitive to molecular weight. Alternatively, the unsaturation sites may be subjected to grafting and crosslinking reactions which raise the molecular weight and undesirably stiffen the polymer making it unprocessable or ineffective as an adhesive. Hydrogenating the conventional unsaturated base polymers creates a nonpolar polymer which, although more stable, is difficult to tackify with resin additives and which is therefore inferior to conventional polymers in some applications, including some pressure sensitive adhesives applications.
Conventional saturated block copolymers designed for use in adhesive formulations are triblock copolymers wherein the terminal blocks are hard phase polymers such as polystyrene blocks. Often, such hard phase terminated block copolymers, especially those with saturated rubber blocks, lack the tackiness and peel adhesion of unsaturated block copolymers in adhesive formulations. Blending hard phase terminated block copolymers with diblock copolymers improves tackiness and peel adhesion but at the cost of decreasing shear properties such as holding power and shear adhesion failure temperature (SAFT).
The present invention offers a solution to some of these problems without sacrificing the adhesive qualities of unsaturated block copolymers. It does so by providing a polymer which has at least one hydrogenated conjugated diene block on the end of the polymer.